1. Field of the Invention
The present invention relates to an apparatus for uniformly dispersing paper raw material that can be utilized in various twin wire formers of a paper making machine.
2. Description of the Prior Art
One known wire former making use of two wires which provide excellent fiber dispersion in paper is a Bel-Baie former (vertical type gap former) manufactured by Beloit Corp. of Beloit, Wis. shown in FIG. 11. However, this former has a shortcoming in that a yield retention is poor. More particularly, in this former, after a raw material liquid has been sandwiched between two wires consisting of an upper wire a and a lower wire b, if dewatering is carried out while rubbing the wires with a fixed member (shoe blades c, suction boxes d, vacuum deflector e and suction couch roll f), then white water having a high concentration will flow out of the wires.
In order to obviate this shortcoming, it is only necessary to reduce the fixed members which perform dewatering by rubbing the wire surface and to carry out dewatering by means of members which travel along with the wires, such as rolls. A Bel-Baie former constructed in such a manner is shown in FIG. 12.
Normally in a Bel-Baie former, the former is operated in such manner that a water level is located above the suction box d. However, if the fixed dewatering devices are reduced by decreasing the numbers of the shoe blades c and the suction boxes d as compared to the arrangement shown in FIG. 11 and also by omitting the vacuum deflector, the water level is located at a position on the suction couch roll f as shown in FIG. 12. The retention is improved, but the fiber dispersion tends to be degraded.
It was previously discovered through research conducted by the inventors of this invention that the fiber dispersion in a twin wire former can be greatly improved by decelerating and accelerating in the direction of travel, a raw material liquid without stirring the raw material liquid by applying a pressure gradient to the raw material liquid at the portion of sandwiching the raw material is sandwiched between two wires or after at a portion before which the raw material liquid has been sandwiched.
An appropriate value of this pressure gradient for carrying out deceleration and acceleration will vary depending upon a paper making speed, a weight of paper per unit area, compounding of the raw material, a concentration under the given state and the like. In the known twin wire former in the prior art, improvements in the fiber dispersion were achieved by applying a pressure variation to the raw material liquid by means of shoes, a foil, a roll suction box, etc., but it was almost impossible to adjust the pressure variation to an appropriate value according to the paper making condition.